A generally known arrangement of a joint coupling for a hydrostatic piston machine has a ball pivot imbedded in a ball recess. In order to minimize the contact pressure between the ball pivot and the ball recess, both spherical surfaces of the ball pivot and the ball recess are manufactured in strict tolerances of their diameters which involves high manufacturing costs. Furthermore this arrangement is not adequate to reduce significantly the contact pressure between the spherical surfaces. In fact, especially due to technological reasons, in order to permit a circulation of fluid between the cylinders and the swash plate in any relative angle of inclination of the swash plate and the cylinder block, the ball pivot ends at a planar surface and the ball recess ends at a rotational recess, which is generally cylindrical, so the contact areas and consequently the most loaded areas of both spherical surfaces are on an edge, either on the circular edge of the rotational recess of the ball recess or on the circular edge of the planar surface of the ball pivot. Contact loads on edges are thus unfavourably high and consequently damages can occur due to unacceptable stresses.
An arrangement of a joint coupling for a hydrostatic piston machine is known by EP0763657 and has a ball pivot of the axial extension of a piston imbedded in a spherical ball recess of a slipper, the bottom of which presents a conical surface continuously connected to the spherical surface. Although this arrangement partially decreases the contact pressure between the ball pivot of the piston and the ball recess of the slipper, accuracy requirements and costs for the manufacture of this arrangement are as high as in the previously known arrangement. In this case when tolerances are large, the diameter of the contact area can be reduced and loads consequently increased. Moreover, this arrangement becomes less efficient as the angle of inclination of the slipper axis with respect to the piston axis increases, as a consequence of the eccentric influence of a force applied from the piston towards the contact circle defined by the intersection of the spherical surface of the ball pivot and the conical surface of the ball recess in the slipper. When this angle of inclination is increased, the axial force transmitted from the piston to the transmission member is considerably off-centred with respect to the joint surface of the ball recess. Accordingly, the contact area on the annular surface around the circle defined by the intersection of the spherical surface of the ball pivot and the conical surface of the ball recess is no longer complete, but is limited to a portion of this annular surface. Resulting stresses are consequently increased.
U.S. Pat. No. 6,024,010 discloses another known arrangements of joint couplings for an axial piston machine have a spherical ball recess, which receives a shoe (or slipper). In a first embodiment, the functional surface of the shoe is created by a substantially spherical surface having a radius of curvature equal or slightly smaller than that of the recess, a top portion of said substantially spherical surface having a larger radius of curvature. In a second embodiment, this functional surface has a generatrix formed by two eccentric arcs of circles, having a radius smaller than the radius of the recess and intersecting on an axis of symmetry at the top portion. U.S. Pat. No. 6,168,389 discloses a joint coupling for which the functional surface of the shoe (or slipper) is created by the rotation of a part of an ellipse around an axis of rotation of the shoe (or slipper).
The disadvantage of the arrangements of U.S. Pat. No. 6,168,389 and of the second embodiment of U.S. Pat. No. 6,024,010 is an unacceptably large radial clearance on both sides of the contact area between the respective functional surfaces of the ball recess and of the shoe (or slipper), which prohibits an application of these arrangements for a transmission of a radial force by means of the joint coupling if the axial force is not sufficient to keep the two adjacent surfaces in contact, in which case the radial clearance allows the shoe to move in the radial direction. Consequently, impacts due to vibrations may occur and damage the machine. In the arrangement of the first embodiment of U.S. Pat. No. 6,024,010, the disadvantage is that there is still an edge effect because the intersection of two spheres the centers of which are on the same axis is an edge.